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pressure cylinder. There are two cross exhaust valves, one on each exhaust pipe, affording very close adjustment. The high-pressure cylinder is equipped with cut-off valves and one of the others is provided with cushion ports and valves. The economy is very high, a large part of the expansive force being utilized. Disadvantages are the great number of cylinders, stuffing boxes, and valves, the large floor space required and also the room needed for taking out the rods. Advantages over crank and flywheel engines are the simplicity of the valve motion, the "light foundation required, the low cost of repairs and attendance and the uniform discharge. Figure 4 shows a longitudinal section through the steam end, fitted with rotary valves. Adjustment for speed is usually made by hand
FIG. 4.—Triple-expansion pump, telescope pattern.
throttle valve, but a Mason speed governor controlling a balanced valve in the steam pipe may be attached. To prevent racing in case of accident to the discharge main an automatic stop valve, normally held open by the water pressure, is recommended. This type of pump should be used when the resulting saving in fuel justifies the increased initial cost.
High-duty pumps are equipped with steam cylinders in which the steam is used expansively. As the load is constant throughout the stroke and the steam piston force diminishes during the period of expansion, some compensating device is necessary to equalize the forces. This may be a fly wheel, an auxiliary
piston, a weight or a compensating gear.
The compensating device used in the Worthington high-duty direct-acting pump consists of two oscillating hydraulic cylinders, arranged so as to take up the excess of energy developed during the first half of the stroke and return it during the second half, where there is normally a deficiency. To reduce the oscillating masses, high water pressure (600 Ib.) is employed. This is furnished by an intcnsifier which has a large air cylinder supplied from the air chamber and a small hydraulic ram.
The low-duty triple-expansion pump has taken the place of the high-duty compound pump and only triple-expansion high-duty pumps are now built. They give the same duty as crank and flywheel triple-expansion pumps, have greater flexibility, require lighter foundations and the valve motion is simple. Adjustment for speed is made by hand throttle valve which, is satisfactory because the load is constant.
Power pumps are pumps having plungers operated mechanically from a rotating driving mechanism. There may be from one to five plungers, arranged